Graphene has attracted a great deal of scientific interest in later years owing to its unique properties, such as large thermal conductivity and excellent electronic transport properties. Theoretical and experimental studies identified its outstanding mechanics, electricity, chemicals, thermal stability, specific surface areas, mobility of charge carriers, and thermal conductivity. Many prospective applications of graphene are being actively investigated at present. However, the actual implementation of graphene in technological uses will depend critically on the development of appropriate methodologies for its mass production.
In this regard, one of the most promising approaches is based on the exfoliation of graphite. However, the above method cannot be mass produced due to its low yield. Furthermore, some methods were provided to put graphite powder or graphite fiber into a solution mixed with the strong oxidant such as sulfuric acid and nitric acid, and then each layer of the graphite will be oxidized to graphite oxide. The graphite oxide is washed to adjust the pH until 7 by water and dried in an oven at 1100˜1250° C. to let the graphite oxide quickly expand and exfoliate to form the two-dimensional graphene oxide. In other case, the graphite powder can be oxidized by sulfuric acid and nitric acid to form graphite oxide. And then, the graphite oxide is washed by DI water to obtain expanded graphite oxide. After processing the expanded graphite oxide at different temperatures of 600° C. and 1500° C., the expanded graphite oxide will be dispersed in the water, exfoliated by ultrasonic oscillation and ground to form the graphene oxide at nanometer degree. However, the above method, which is performed by mixing the strong oxidant and heating, is too complicated to be mass produced.
Hummer's method is provided in addition to the abovementioned methods. It is to fabricate the oxidized graphite and spin-coat the oxide on a silicon oxide substrate. And then, the substrate will be processed a reduction reaction for 20 hours to obtain the graphene. However, this method only can fabricate the graphene with the functional group on its surface. On the other hand, the other method is also provided to sputter and deposit nickel on the SiO2 substrate to form a catalyst layer with a thickness of 100 nm. And then, carbon source such as ethylene will be loaded inside and split to form carbon to deposit on the surface of the catalyst layer. Finally, the substrate will be put into 0.1M HCl to corrode nickel layer to obtain the graphene. Although this method can form fewer layers of graphene on the substrate with the large area, the high temperature such as 950° C. is needed to split the carbon source.
To sum up, although the graphene can be fabricated by exfoliating, grinding, cutting open nanotubes, oxidation-reduction reaction, hetero-epitaxy or mixing the acids and heating, the abovementioned methods always have some limitations, such as low yield, complicated procedure or toxic agent, so that the above methods cannot be utilized to mass produce graphene.